1. Field of the Invention
The present invention relates to a vehicle antenna system and particularly to an improved vehicle antenna system which can efficiently detect broadcast waves received by the vehicle body and deliver the detected signals to various receivers on board the vehicle.
2. Description of the Prior Art
Antenna systems are essential in vehicles for positively receiving various broadcast waves such as radio and TV waves, and communication waves such as car-telephone waves. A vehicle antenna system also is very important for a citizen band transceiver which effects communication between a vehicle and another stationary or moving station. Such vehicle antenna systems are anticipated to perform very important functions with communication instruments to be standardly mounted in future vehicles.
In the prior art, a generally used and known antenna system is the pole type antenna system which extends outwardly from the vehicle body and which provides favorable reception performance in its own way. However, the pole type antenna is nothing less than an obstacle vehicle design.
Furthermore, the pole type antenna is frequently bent or damaged intentionally or carelessly. In addition, the pole antenna tends to produce unpleasant noises when a vehicle on which the pole antenna is mounted runs at high speeds. For these reasons, it has been largely desired to omit the pole type antenna from the vehicle.
Recently, there have been allocated additional frequency bands for broadcast or communication waves which are to be received by vehicle receivers. A plurality of antenna systems are required to receive waves in the respective frequency bands. This further damages the aesthetic appearance of the vehicle. Moreover, the plurality of antenna systems produces an electric interference between antennas thereby degrading reception.
Some efforts have been made to remove or conceal the pole type antenna. For example, an antenna wire is applied to the rear window glass of a vehicle.
Another proposal has been made to detect surface currents induced by receiving broadcast waves at the body of a vehicle. This apparently should provide the most positive end efficient means for receiving broadcast waves. However, experiments have indicated that such a proposal was not as superior as was expected.
A first problem produced when the surface currents are to be used to receive broadcast waves at vehicle receivers is that the surface currents are not as strong as expected. In the prior art, the surface currents are mainly utilized at the front roof of the vehicle body. In spite of this, no sufficient signal level could be obtained to utilize the surface currents.
A second problem is that the surface currents include noises at a very large rate. Such noises are mainly produced by the ignition system and charging regulator system of a vehicle engine and thus cannot be removed as long as the engine runs. Therefore, the clear reception of broadcast waves could not be realized at all.
Under such disadvantageous conditions, some other proposals have been made. For example, Japanese Patent Publication 53-22418 discloses an antenna system utilizing currents which are induced on the body of a vehicle. The antenna system comprises an electric insulation provided at a portion of the vehicle body in which currents are concentrated. The antenna system also comprises a sensor for directly detecting the currents between the opposite ends of the electric insulation. In such an arrangement, it is sure that the antenna system can detect practicable signals which are superior in S/N ratio. However, the antenna system requires a pickup structure which must be disposed in a notch intentionally formed on a portion of the vehicle body. This proposal cannot be applied to mass-production vehicles.
Japanese Utility Model Publication 53-34826 discloses another antenna system comprising pickup coil means for detecting currents on the pillar of a vehicle. This antenna system is disadvantageous in that it can completely be contained within the vehicle body. However, the antenna system requires an undesirable arrangement wherein the pickup coil must be disposed near the pillar in a direction perpendicular to the longitudinal extent thereof. Moreover, such an arrangement cannot obtain any practicable antenna output and is therefore at best a poorly conceived idea.
As can be seen from the foregoing, the prior art does not provide any proper construction and arrangement of a pickup device which are required to effect the efficient detection of currents flowing on the vehicle body and to obtain a practicable S/N ratio. Rather, experiments show that the antenna system utilizing currents on the vehicle body is probably ineffective in principle.
Recently, TV sets are increasingly being mounted on vehicles not only to receive TV waves but also to display various data relating to the vehicles. In such vehicle TV sets, TV wave signals are separated into image signals and voice signals. When the vehicle is at zero speed, both the image and voice signals are used in the TV set. When the vehicle runs at speeds above a predetermined level, only the voice signals are outputted from the TV set.
Such vehicle TV sets have a problem in that the quality of the image can be reduced since the state of reception is changed depending on the condition of the vehicle.
In order to overcome such a problem, the prior art vehicle TV set comprises a plurality of TV antennas controlled by a diversity receiving system which can select optimum TV antennas depending on the state of reception.
In such a case, a plurality of TV antennas are disposed on the body of a vehicle at various preselected locations. The diversity receiving system is electrically connected with the TV antennas such that they can selectively be used depending on the state of reception for image signals separated from received signals. The diversity receiving system is adapted to compare the image signal level with a reference level in synchronism with the vertical blanking interval of the image signals to select optimum TV antennas.
The diversity receiving system also has a problem in that the construction and arrangement of a pickup device required to efficiently detect currents induced by TV waves at the vehicle body and also to obtain a practicable S/N ratio cannot properly be established. Particularly, a high frequency pickup used as a TV antenna does not have a good high frequency directional pattern. And yet, multi-path noises tend to be produced during reception of FM waves having a high frequency belonging to the VHF band.